In recent years, humidity measurements have played an ever increasing role in industrial, laboratory and process control applications by enhancing the quality of products produced while simultaneously effecting significant economies.
The technology known as "chilled mirror hygrometry" was introduced some three decades ago and has resulted in the most accurate, stable and repeatable dewpoint measurement instruments to be sold commercially.
In this technology, there is typically employed an optically smooth surface which is continuously cooled by means of a thermoelectric cooling device to a temperature known as the dewpoint, the temperature at which a sample of air becomes saturated and produces dew or mist. This process involves the lowering of the temperature of the mirror at a precisely controlled rate until the formation of dew is detected. Before the dew so formed is able to form a continuous layer, the mirror is heated and the dew thereon evaporated. Accordingly, the mirror surface is essentially in a dry state and includes a dew layer for a very limited time period during which the dewpoint measurement is made.
Although this technology has been used successfully for many years, it has been recognized that any retention of a dew layer on a mirror surface tends to encourage airborne contaminants to adhere to the mirror. Such contamination impairs the reflective performance of the mirror, so resulting in the introduction of measurement errors unless a continuous cleansing procedure is employed. Heretofore, it has been common practice to effect this cleansing by wiping the optical surface with a smooth cloth such as a chamois or the like. Although such techniques have been used with a certain level of success, workers in the art have continued in their efforts to develop new techniques for reducing mirror contamination.